Video encoders encode a video bit-stream according to a predetermined standard syntax, such as MPEG-2 or Advanced Video Compression (AVC). An encoder generating a compressed video bit-stream makes a number of choices for converting the video stream into a compressed video bit-stream that satisfies the quality of service and bit-rate requirements of a channel and media. Often, video data is transferred over a communication channel, with limited bandwidth, thus requiring compression on the encoder side.
One of the problems associated with compression is that it is a lossy process. Since compression discards some information in the video data, the discarded information is not recoverable. Other problems arise as a result.
To illustrate, consider, for example, an MPEG encoding process where video data comprises a series of frames. Each frame comprises two-dimensional grids of pixels. The two-dimensional grids are divided into smaller blocks of pixels, such as 16×16 blocks of pixels. These 16×16 blocks of pixels form a data structure known as a macroblock. The macroblock includes luminance and chrominance information, as well as additional parameters, including motion vectors. Each macroblock represents image data in a 16×16 block area of the image.
The data in the macroblocks is compressed in accordance with algorithms that take advantage of temporal and spatial redundancies. The macroblocks representing a frame are grouped into different slice groups. The slice group includes the macroblocks, as well as additional parameters describing the slice group. Each of the slice groups forming the frame forms the data portion of a picture structure. The picture includes the slice groups as well as additional parameters that further define the picture.
The pictures are then grouped together as a group of pictures (GOP). The GOP also includes additional parameters further describing the GOP. Groups of pictures are then stored, forming what is known as a video elementary stream (VES). The VES is then packetized to form a packetized elementary sequence. Each packet is then associated with a transport header, forming what are known as transport packets.
The transport packets can be multiplexed with other transport packets carrying other content, such as another video elementary stream or an audio elementary stream. The multiplexed transport packets form what is known as a transport stream. The transport stream is transmitted over a communication medium for decoding and displaying.
A group of pictures comprises reference pictures I and P, and bi-directional pictures B. Pictures coded using temporal redundancy with respect to exclusively earlier pictures of the video sequence are known as predicted pictures (or P-pictures). Pictures coded using temporal redundancy with respect to earlier and/or later pictures of the video sequence are known as bi-directional pictures (or B-pictures). Pictures not coded using temporal redundancy are known as I-pictures. In the MPEG-2 standard, I-pictures and P-pictures are also referred to as reference pictures.
A GOP generally starts with an I picture, followed by B and P pictures, up to the end of the GOP, where another GOP begins with another I pictures. Often, GOPs represent adjacent parts of a video sequence. In each GOP, B or P pictures are typically encoded using the I pictures associated with the GOP. When the next GOP begins, the pictures are encoded using the new I. Problems arise because of the time gap between one I picture and the next I picture. The pictures in one GOP are coded based on the I picture in the beginning of the GOP, and all of a sudden the pictures in the next GOP begins with an I picture that is relatively later in time from the earlier I picture. While in an original video sequence the change from the first I picture to the next I picture has a smooth transition, coding the B pictures based on the preceding I picture causes visible interruption in the transition between consecutive GOPs. This is known as “I-clicking” and may be seen in areas within a picture with a lot of detail such as, for example, a picture with wood texture.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.